HT transmissions need protection from legacy stations (STAs), since legacy STAs using contention will not be able to decipher HT physical layer (PHY) packet data units (PPDUs), and thus the legacy STAs may not interpret the medium as being busy. Similarly, HT transmissions of a given HT STA require protection from other HT STAs in the case where the HT PHYs are incompatible.
As shown in FIG. 1, each legacy PPDU 100 comprises a legacy PHY preamble/header 105 followed by a legacy PHY payload 110, which is typically a medium access control (MAC) packet data unit (PDU). As an example, FIGS. 2A and 2B show that an HT device (i.e., Type A) and another HT device (i.e., Type B) have different PHY preambles 115, 120. The Type A device has both a legacy preamble 105 as well as a new preamble 115 for MIMO capability as part of a PHY header, whereas, the Type B device only has a new PHY preamble 120. However, the Type B device can decode the legacy preamble 105. The HT PHY preambles 115, 120 of both the Type A and B devices are different. In this case, the two devices will not be able to communicate with each other. Also, the Type A device is inefficient in absence of any legacy devices. Two considered scenarios (1 and 2) are described below.
Scenario 1 uses at least three different types of devices which operate with different/incompatible PHY layers: 1) legacy devices which transmit packets with only a legacy preamble 105, as shown in FIG. 1; 2) HT Type A devices which transmit PPDUs with Type A HT preambles 115, as shown in FIG. 2A; and 3) HT Type B devices which transmit PPDUs with Type B HT preambles 120, as shown in FIG. 2B.
Scenario 2 also uses at least three types of devices: 1) legacy devices which transmit packets with just a legacy preamble 105, as shown in FIG. 1; 2) HT devices which transmit mixed-mode packets, (e.g., a PHY preamble comprising a legacy preamble and an HT preamble), as shown in FIG. 2C; and 3) HT devices which transmit packets withjust HT PHYpreambles, (i.e., a Green Field (GF) PHY preamble 120 and HT preamble 115), as shown in FIG. 2D.
Thus, different types of HT devices may operate using mutually incompatible PHY layers. For example, the receiver of a first type of HT STA may not be able to decode packets transmitted by a second type of HT STA, and vice versa.